1. Field of the Invention
The present invention relates to a dynamic pressure bearing device in which one of a shaft and a bearing member is rotatably supported by the other with dynamic pressure generated by lubricating fluid.
2. Description of Related Art
In recent years, a dynamic pressure bearing device, in which a rotary shaft is supported by a dynamic pressure generated in a lubricating fluid, has been developed as a bearing device that can rotate a rotary body at a high speed and high accuracy for various kinds of rotational driving devices. In such a dynamic pressure bearing device, a bearing having a thrust bearing part SB structured as shown in FIG. 4, for example, has been proposed in order to make the entire device thinner. In the thrust bearing part SB, a rotary member (rotary hub) 3 is attached to a rotary shaft 2, which is supported in a freely rotatable manner by a dynamic pressure bearing member (bearing sleeve) 1. An inner end surface in the axial direction (under surface in FIG. 4) at the center area of the rotary member 3 is positioned opposite and in close proximity in the axial direction to an outer end surface (upper end surface in FIG. 4) of the dynamic pressure bearing member 1, thereby forming the thrust bearing part SB at the thrust opposing area.
In the interior of the thrust dynamic pressure bearing part SB, a lubricating fluid is filled and spiral-shaped dynamic pressure generating grooves, for example, are formed in the circumferential direction as a dynamic pressure generating means for the lubricating fluid, so that the pressurizing effect of the dynamic pressure generating grooves causes dynamic pressure to be generated in the lubricating fluid and thereby yields a predetermined levitation force in the axial direction.
Also, two radial bearing parts RB and RB are formed in the axial direction at a radial opposing region which is formed by opposing an inner circumference surface of the dynamic pressure bearing member 1 to an outer circumference surface of the rotary shaft 2. A lubricating fluid is filled in a bearing space for each of the radial dynamic pressure bearing parts RB continuously from the thrust dynamic pressure bearing part SB. As a dynamic pressure generating means for the lubricating fluid, for example, herringbone-shaped dynamic pressure generating grooves are provided in the circumferential direction, and dynamic pressure is generated to the lubricating fluid by means of a pressurizing effect of the dynamic pressure generating grooves to obtain a radial levitating force.
In the dynamic pressure bearing device, the bearing space is formed in a continuous manner from the two radial bearing parts RB and RB to the thrust dynamic pressure bearing part SB, and the lubricating fluid is continuously filled in the continuous bearing space.
In such dynamic pressure bearing devices, the groove shape (groove length) of the dynamic pressure generating grooves provided on the radial bearing part RB sometimes becomes unbalanced in the axial direction due to processing errors at the time of manufacturing. As a result, for example, as shown with the lengths of the arrows in the lower radial bearing part RB in FIG. 5, the pumping effect forces P1 and P2 to become in an unbalanced state (for example, P1>P2) in the axial direction. That causes to affect the thrust levitated amount in the thrust dynamic pressure bearing part SB.
The variation of the levitated amount in the thrust direction due to the unbalance in the radial bearing part RB is generally proportional to the variation of the viscosity of the lubricating fluid. As shown in FIG. 6, for example, the variation of the levitated amount in the thrust direction (vertical axis) with respect to the temperature variation (horizontal axis) becomes larger at a region on a lower temperature side. FIG. 6 shows a graph in which the weight (g) of the rotary body is a parameter, and it is understood that the variation of the thrust levitated amount is easy to be affected particularly in small and lightweight rotational driving devices, which has a light-weight rotary body. More specifically, as shown in FIGS. 7 and 8, the tilt of the variation curve of the thrust levitated amount (vertical axis) of the rotary body becomes larger as the weight of the rotary body (horizontal axis) becomes smaller and the thrust levitated amount becomes sharply larger particularly as the temperature becomes lower.
The variation of the levitated amount in the thrust direction due to the unbalance in the radial bearing part RB has caused a significant problem for rotational driving devices that drive a smaller and lighter rotary body with a small-sized spindle motor in recent years. For example, in a hard disk drive device (HDD) which drives a hard disk for information recording, a head may experience an impact on the hard disk especially under cold temperatures.
It is conceivable that the variation of the thrust levitated amount in the dynamic pressure bearing device is suppressed by means of a magnetic attracting means by use of a permanent magnet or the like. However, when the magnetic attracting force is too strong, the minimum speed required to levitate in the thrust direction increases, and the time period when the rotary body is sliding on the dynamic pressure bearing member is increased at the time of starting or stopping. Therefore, wear of the dynamic pressure bearing member is rapidly generated to cause poor rotation. Also, when the unbalanced quantity of the radial bearing part RB is large, there may be a possibility that the thrust levitation in the thrust bearing part SB is not obtained at all.